1. Field of the Invention
The present invention relates generally to controllers for AC driven loads. It relates specifically to pulse width modulated (PWM) controllers for variable speed induction motors.
2. Discussion of the Related Art
Reduction of AC power to an inductive load is often problematic due to lack of a path for the necessary discharge current flow from an inductor when the current is interrupted. Also, power reduction, or interruption, may introduce undesired noise into the line. For example, a traditional dimmer switch utilizing triacs may introduce unwanted noise into the line. Variacs are good for reduction of AC power but are too expensive and bulky for many applications.
As another example, control of induction motors to achieve variable speeds is problematic due to increased noise and inefficient operation. Further, when line power is switched off to the load back emf from inductance may damage sensitive components in the power controller if not properly channeled. Ideally, an "electronic variac" being reasonable in size and cost would solve many practical problems for adjustable power delivery to a load.
PWM controllers have been proposed in the past for use with induction motors to resolve many of the shortcomings inherent in trying to use induction motors in variable speed applications. See, for example, An Approach to Realize Higher Power PWM AC Controller, Enjeti and Choi, Applied Power Electronics Conference and Exposition, 1993. APEC '93 Conference Proceedings, 1993, Eighth Annual, pages 323-327 (IEEE: 0-7803-0982-0/93). Enjeti and Choi teach that while the PWM controlled AC controller for regulating power to the motor will decrease unwanted harmonics, commutation problems for controlling switching of inductive load current can be difficult. They propose a four step switching strategy for control of two bidirectional semiconductor switches routing the load current. An experimental controller is detailed as a proof of concept vehicle for steady speed, generalized, inductive load applications.
Four-step switching is a method of controlling bidirectional switches in an alternating current (AC) application so that back electromotive force (emf) from an inductive load is never presented with an open circuit thus allowing it to increase to a large value that destroys the circuitry.
Referring to FIG. 1, the bidirectional switches S1, S2 are configured from a pair of inverse serial connected MOSFET transistors S1A, S1B and S2A, S2B, respectively. The inverse parallel diodes D1A, D1B, D2A, D2B inherent to each MOSFET are shown because they are essential in providing a circulating current path when the inductive load is switched. The complete switch for an inductive load L is comprised of a series switch S1 and a shunt switch S2. In general, the series switch S2 provides current to the inductive load L and the shunt switch S1 provides a circulating or freewheel path for the current in the inductive load L when the series switch S2 is turned off.
It can be seen that conventional switching methods have problems in the configuration of FIG. 1. If a dead band in switching time is provided between, for instance, the turn off of S1 and the turn on of S2, then the back electromotive force would increase during the time that both switches were off and potentially destroy the circuit. If some overlap in switching time is allowed then the shoot through current as S1 and S2 are connected across the AC supply is potentially destructive.
To illustrate the operation of the four-step switching method, one complete switching transition will be described. With reference to FIGS. 1 and 2 consider the transition between S1 on and S2 off to S1 off and S2 on, when the polarity of the AC supply is as shown.
Initially S1A and S1B are on and S2A and S2B are off.
(1) S2B is turned on, nothing happens because S2A is off. PA1 (2) S1A is turned off, the potential on S1A source rises until the on S2B forward biases the inverse parallel diode of S2A. This provides a circulatory path for the inductive load current and the back electromotive force is trapped at slightly more than line potential. PA1 (3) S2A is turned on, nothing significant happens because S2A's diode is already conducting. PA1 (4) S1B is turned off, nothing happens because S1A is already off.
The transition is now complete. It will be appreciated that the switching sequence must be different if the line polarity is opposite and for that reason detection of line polarity must be provided. For a complete description of all transitions and polarities the reader is referred to the Enjeti and Choi article.
However, certain improvements to the Enjeti and Choi four step commutation controller were deemed necessary to make their controller scheme a practical reality for commercial applications of AC power control such as dimmer switches or blower motors of heating, ventilation and air conditioning (HVAC) systems where a variable speed, low noise, long life, fractional horsepower motor could greatly improve the efficiency of HVAC systems. It is these improvements which are the subject of the present invention.